Generate variant build context folder named after tag instead of organized by distro, when distro is specified.

[leojonathanoh]

Current

Generate variant build context folder is organized by $VARIANT['distro'].

Build context folder is /variants/<distro>/$VARIANT['tag_without_distro'].

Expectation

Generate variant build context folder is named after the $VARIANT['tag'].

Build context folder is simply /variants/$VARIANT['tag'].

Discussion

E.g. when a variant distro is specified, the variant build context folder looks like:

└── variants
|   └── $VARIANT['distro']
|       └── $VARIANT['tag_without_distro']
|           └── Dockerfile

when it would be more intuitive to simply be

└── variants
|   └── $VARIANT['tag']
|       └── Dockerfile

Benefits

• Single level tree is easier to follow
• Single level tree matches flat hierarchy of image registries e.g. docker hub, which does not use a tree structure
• Intuitive to browse for build context folders, since they are named after tags
• CI files and CI build job logs show that the build context folder matches the tag, improving traceability and debuggability
• A tag with $VARIANTS['distro'] not being in the prefix or suffix position will not generate an build context folder of an unpredictable name. For instance, a tag such as foo-<distro>-bar will no longer be generated as /variants/<distro>/foo-bar, which is unintuitive, since the tag